This series and parallel sensor circuit was found in Popular Electronics from the 1990s. Any SCR can be used and the supply voltage depends on the load.
This circuit was found in a 90s Popular Electronics. The frequency is determined by R3 and C1 that...
This circuit also detects IR, providing LED indication. The circuit is used to indicate humidity in...
This PNP transistor power stage was found in the May 1969 issue of Radio Electronics magazine. The circuit...